Polyalkylene terephthalate resins are used for manufacturing fibers, films, and molded articles that have excellent physical properties such as high wear resistance, durability, and heat resistance. The physical properties can be further improved by incorporating reinforcing materials such as glass fibers into polyalkylene terephthalate resin compositions.
An important class of polyalkylene terephthalate resins are the PCT resins based on reactions between 1,4-cyclohexanedimethanol and terephthalic acid or suitable synthetic equivalents. Glass fiber reinforced PCT compositions having high crystallinity have excellent properties with regard to strength, stiffness, and heat resistance. These PCT compositions are particularly suitable for the manufacture of heavy duty molded parts for use in the automotive and electronic industries.
High crystallinity is essential to ensure adequate hardness, strength, and heat resistance at elevated temperatures. In addition, high crystallinity should be arrived at as rapidly as possible to achieve optimum material properties and rapid molding cycles. To ensure adequate crystallinity, PCT compositions generally require mold temperatures of 150.degree. C. or higher. High mold temperatures are possible only with sophisticated molding equipment and require specialized heating means, such as, for example, oil bath heating units which are beyond the equipment capabilities of many mold shops which have only water heated molds. Water heated molds are normally capable of temperatures no higher than about 110.degree. C. Oil heating units, in contrast, are expensive to purchase and to operate.
Crystallization aids have been suggested as a method of promoting crystallization at lower mold temperatures. The use of ester compounds as crystallization aids is described, for example, in European patent application 387,398 and in U.S. Pat. Nos. 3,565,852, 4,223,125 and 4,223,113. European patent application 387,398 discloses the use of aliphatic polyesters as crystallization aids for polyethylene terephthalate compositions. The European application states that faster crystallization rates result from the reaction between the aliphatic polyesters and polyethylene terephthalate. PCT is specifically excluded. U.S. Pat. No. 3,565,852 discloses the use of polyesters with at least partial aliphatic character as crystallization aids. Such polyester crystallization aids are likewise limited to polyethylene terephthalate compositions.
U.S. Pat. No. 4,223,125, issued Sep. 16, 1980, discloses polyalkylene terephthalate compositions which contain an aromatic carboxylic acid ester as a crystallization aid. U.S. Pat. No. 4,223,113, issued Sep. 6, 1980, discloses polyalkylene terephthalate compositions which contain an oligomeric polyester having a maximum number-average molecular weight of 6000. U.S. Pat. No. 4,223,113 specifically teaches away from using oligomeric polyesters having molecular weights in excess of 6,000. In column 8, lines 56 to 63, it states that the oligomer's effectiveness in promoting crystallization decreases with increasing molecular weight, thus, oligomers having molecular weights less than 3,000 are preferred.
In contrast, the present inventor has discovered that the addition of an aliphatic polyester having a number-average molecular weight of at least 8,000 and preferably greater than 22,000 allows a lower mold temperature to be used without impairing the crystallization of poly(1,4-cyclohexylenedimethylene terephthalate) compositions. In addition, the degree of crystallinity necessary to ensure adequate hardness, strength, and heat resistance at elevated temperatures is achieved.